For the narrow-line region the Balmer decrement, H\(\) /H\(\) , is an easy-to-use reddening indicator. The intrinsic ratio is expected to be close to a Menzel & Baker Case B value of 2.7–3.1 (see Table 4.4 of [1]}). Comparison with other NLR reddening indicators [2]}, [3]}, [4]} supports this. However, as noted, for the broad-line region there have long been doubts about whether Balmer-line ratios are compatible with simple reddening of case B values. There are two firm observational indications that the intrinsic ratio does depend somewhat on physical conditions. Firstly, the Balmer decrement at a given time becomes flatter with increasing gas velocity [5]}, [6]}. Secondly, reverberation mapping shows that the variability-weighted radius of the higher-order Balmer lines is progressively somewhat smaller as one goes up the Balmer series (e.g., [7]}). These results point to the Balmer decrement getting flatter with increasing density and temperature as is expected closer to the centre of the AGN. This is in agreement with recombination theory which predicts that the Balmer decrement gets flatter with increasing temperature and increasing density. This can be seen in Table 4.4 of [1]}. Figure 4 of [9]} shows the effect for higher densities. Despite these expected and observed effects, the velocity-integrated line ratios of the bluest, and presumably least-reddened, AGNs nevertheless imply an intrinsic H\(\) /H\(\)  ratio of \(\approx 2.8\)  for the BLR as a whole [9]}. This is close to a Case B value. We will therefore adopt H\(\alpha \) /H\(\)  = 2.8 for the velocity-integrated broad lines of NGC 5548.
